1. Field of the Invention
The present invention relates to an electrode plate suitable for a nonaqueous electrolyte secondary battery, e.g., a lithium secondary battery or the like, a method of manufacturing the same, and a secondary battery using the same. More specifically, the invention relates to (1) an electrode plate with an insulating jointed to the plate effectively and accurately in a predetermined position to prevent short circuits, (2) a method of manufacturing the same, and the like.
2. Description of the Background Art
Because of the rapid growth and development of portable electronic devices, the specification requirements for a battery used for such products have become more and more exacting. In particular, a compact low-profile battery with a high capacity, excellent cycle character, and stable performance is absolutely desirable. In the field of secondary batteries, lithium secondary batteries have generated more interest than other batteries because of their high energy density, and as a result their market share continues to grow.
Such a lithium secondary battery comprises a negative electrode having a strip-shaped negative electrode collector made of copper foil or the like and a negative electrode active material coating applied on each side of the negative collector, a positive electrode having a strip-shaped positive electrode collector made of aluminum foil or the like and a positive electrode active material coating applied on each side of the positive collector, and a separator made of fine porous polypropylene film or the like, the negative and positive electrodes being coiled together via the separator in cylindrical or elliptic cylindrical form, with the separator providing electrical insulation between the negative and positive electrodes. In the case of a square battery, such a coiled electrode body is pressed or compressed into a flat shape, and a negative lead is welded to a predetermined part of the negative electrode, while a positive lead is welded to a predetermined position of the positive electrode, both of these electrodes being accommodated in a casing having a predetermined shape.
The above-mentioned flat shaped coiled electrode body is normally produced in the following manner.
First, a strip-shaped negative electrode collector is intermittently coated on both sides thereof with a negative electrode active material mixture in its longitudinal direction, and then the collector is processed to form a negative electrode member of predetermined thickness and width having a number of negative electrodes arranged in sequence. Likewise, a strip-shaped positive electrode collector is intermittently coated on both sides thereof with a positive electrode active material mixture in a longitudinal direction, and then the collector is processed to form a positive electrode member of predetermined thickness and width having a number of positive electrodes arranged in series. The negative and positive electrode members so obtained are then fed to a predetermined coiling position, together with the two separators interposed between the electrode members, in a coiled state.
In the coiling position, the negative electrode member, a first separator, the positive electrode member, and a second separator are disposed layer by layer in that order and coiled around a cylindrical or elliptic cylindrical core which resides on the inner side of the negative electrode member. Thereafter, a negative electrode part wound around the core is cut from the negative electrode member, while a positive electrode part wound around the core is cut from the positive electrode member, and parts of the first and second separators used for the single flat coiled electrode body around the core having a length similar to that of the electrode body are cut from the respective separators. Coiling processing is performed to manufacture a number of coiled electrode bodies in turn, each having a substantially cylindrical or elliptic cylindrical shape. It is noted that the negative and positive leads are welded or molded to non-coated parts of the negative and positive members.
The square battery is manufactured through the use of a predetermined press machine which is designed to clamp and press the cylindrical or elliptic cylindrical electrode body in a radial direction to form a flat electrode body.
However, in the formation of such an electrode body, since the negative and positive electrodes coiled around the core are cut out from the respective negative and positive electrode members, the cut edges of the negative and positive electrodes, that is, the cut edges of the negative and positive electrode collectors made of metal form burs. Moreover, in the process, peeling off of the active material and wearing away of the manufacturing device cause conductive particles on the non-coated parts of the active materials to remain on the positive and negative electrodes.
When the elliptic cylindrical electrode body is pressed to form a flat electrode body, the remaining burs or particles produced cause the adjacent separator to break away, thereby establishing electrical continuity between positive and negative electrodes via these burs, causing a short circuit. This short circuit generates extraordinary heat during use of the battery, lowering the capacity thereof, thereby shortening its service life.
To eliminate glitches caused by the burs, a method has been proposed whereby electrode leads are to reside at the center of the coiled electrodes and at the outermost portion thereof, such that electrodes of opposite polarity will not face connecting portions of the electrode leads, or to apply an insulating coating to a portion of at least one positive electrode, a separator and one negative electrode, which portion is opposed to a positive lead. The method of using the insulating coating is most frequently carried out in particular, as disclosed in many patents, for example, in Japanese Unexamined Patent Publication No. 2002-42881 (see page 3, right column to page 4, left column, and FIGS. 1, 2 and 8), and in Japanese Unexamined Patent Publication No. 10-241737(see page 3 to page 4, and FIG. 1). The former patent discloses a tape sticking device used to adhere insulating tape to a portion of at least one positive and one negative electrode and separator, which portion is opposed to the positive lead. FIG. 4 is a perspective view of this tape sticking device. FIG. 5 is a side view explaining the process of securing the insulating tape to a negative electrode member, using the tape sticking device of FIG. 4.
A tape sticking device 60 includes an upper tape sticking mechanism 61, and a controller 62 for controlling this tape sticking mechanism. It should be noted that a lower tape sticking mechanism and another controller for controlling the mechanism are provided opposed to the upper tape sticking mechanism 61 beneath a negative electrode member 71, but they are not shown in FIG. 4.
The upper tape sticking mechanism 61 includes a head 63 for sucking a strip-shaped insulating tape 74, a moving means 64 for moving the sucking head 63 in a predetermined direction, and a cutter 65 for cutting the strip-shaped insulating tape 74. Head 63 is connected via line 63A to a vacuum supply (not shown). The controller 62 is driven to control the whole of the upper tape sticking mechanism 61 based on signals from an active-material detecting sensor 66 and a side-edge detecting sensor 67, and on signals from a rotary encoder 68, thereby causing the insulating 74 cut out to stick to a predetermined position.
In the tape sticking device 60, a negative electrode collector 69 is intermittently coated on both sides with an active material for negative electrode 70, and negative leads 72 are welded in sequence to non-coated areas 69A. This collector is lead by a guide roller 73. In this state, the strip-shaped insulating tape 74 is sucked by the sucking head 63 by back pressure generated in a vacuum duct 63A and transferred onto the negative electrode member 71. And, as shown in FIG. 5(a), with a tip of strip-shaped insulating tape 74 substantially aligned with the moving edge path of the cutter 65, the sucking head 63 is moved to a sticking position by the moving means 64 to abut against an upper surface 74B of the tip of the strip-shaped insulating tape 74.
Thereafter, as shown in FIG. 5(b), the sucking head 63 is moved to a cutting position on one side of the negative electrode member 71 side, causing a cutting position of interest 74C of the strip-shaped insulating tape 74 to substantially align with the moving edge path of the cutter 65. Then the cutter 65 is moved to cut out a strip whose tip is on a sticking position of interest 74D, from the strip-shaped insulating tape 74.
Finally, as shown in FIG. 5(c), the sucking head 63 is moved to above the negative electrode member 71, causing the lower surface 74A of the strip-shaped insulating tape 74D to be opposed to the upper surface of the negative electrode member 71. Subsequently, the sucking head 63 is moved downward, causing the lower surface 74A of the strip-shaped insulating tape 74D, on which an adhesive is applied, to abut against the upper surface of the negative electrode member 71. Then, negative pressure applied to the sucking head 63 is released, so that the strip-shaped insulating tape 74D adheres to the upper surface of the negative electrode member 71.
The tape sticking device 60 is also applicable to a positive electrode member. The positive electrode member has the strip-shaped insulating tape adhering to various kinds of possible short-circuit portions of the positive electrode with respect to its upper and lower sides, including a boundary place between the positive electrode collector and the area coated with a positive electrode active material, a welding place of a positive lead, or the like, for example. Adhesion of the insulating tape to the electrode as described above also effectively helps prevent the occurrence of short circuits due to the presence of conductive particles.
The tape sticking device 60 can cause the insulating tape to adhere in advance to a possible short-circuit portion where a short-circuit may occur between the negative and positive electrodes due to burs on the cut edges of the negative and positive electrode collectors, i.e., on the cut edges of the negative and positive electrodes, when forming the flat electrode body.
However, the insulating tape sticks to the possible short-circuit part such that its longitudinal length is slightly longer than the width of one of the negative and positive electrodes depending on the height of the burs, with one end of the tape in the longitudinal direction protruding outward from one side edge of the electrode, and with the other end thereof in the longitudinal direction protruding outward from the other side edge of the electrode. That is, as shown in FIG. 5(c), the strip-shaped insulating tape 74D has its right and left ends protruding from the negative electrode member 71 by a length L, respectively. Further, it is actually difficult if not impossible to fix or secure the insulating tape having the same dimension as the electrode so as not to protrude from the side edges of the electrode.
For that part of the insulating tape which protrudes from the electrode body, paste material included in the protruding part will adhere or stick to a press machine in the post-process to be carried out by a manufacturing device, e.g., when compressing the electrode body with the insulating tape adhering thereto. This often necessitates cumbersome handling, such as cleaning of the press machine or the like, leading to reduced production efficiency.
When an electrode body is inserted in an exterior can, the paste material of the insulating tape often adheres to the can, making it difficult to insert the electrode body further. Moreover, conventionally, before the coiling process is carried out to make the electrode body after the electrodes are processed to set their width to a predetermined size, an insulating tape must be applied to each electrode plate. This unfortunately results in extremely low efficiency in the production of the electrode body.